1. Field of the Invention
The present invention relates to an optical pickup for writing and reading magneto-optical signals recorded in a magneto-optical recording medium or, more particularly to an optical pickup comprising an optical substrate of a material such as glass, optical crystal or transparent plastic and at least an optical device formed on a surface of the optical substrate.
2. Description of the Related Art
Conventional equipment using a semiconductor laser finds wide applications with computer peripherals, optical communication equipment, etc. One of such examples is a magneto-optical disk device.
A semiconductor laser is used as a laser light source for writing and/or reading signals into and/or from a disk medium. Optical devices including a detector for condensing, splitting and detecting the laser beam and a semiconductor laser are collectively called an optical pickup.
In recent years, an apparatus has been under development, in which a magneto-optical disk device is mounted in a personal computer (PC) for handling a large capacity recording medium. Mounting such an apparatus in a notebook-sized PC requires a magneto-optical disk device which is further reduced in size and power.
A lighter weight and a smaller size of optical parts are also required for effective utilization of the internal space of the magneto-optical disk device.
An application using a hologram is shown in FIG. 1 as an example of an optical system used in a magneto-optical disk device. This optical system is so configured that the light is separated by a hologram for detecting a tracking signal and a condensing signal, thereby realizing integrated functions. In FIG. 1, a laser beam emitted from a semiconductor laser or light emitting diode (LD) 1 spreads at a predetermined angle while being transmitted through a hologram 2 and corrected into parallel light rays by a collimator lens 3. Further, the laser beam is transmitted through a beam splitter 4 and condensed by an objective lens 5 on the surface of a medium, such as a magneto-optical disk 6. Signal light reflected from the surface of the medium 6 is transmitted through the objective lens 5 and partly reflected by the beam splitter 4, and through a Wollaston prism 7 and a condenser lens 8, and is detected as a magneto-optical signal by a magneto-optical signal photo-detector shown to the left of FIG. 1. On the other hand, a component of the light that has passed through the beam splitter 4 reaches the hologram 2 through the collimator lens 3. A part of the laser beam is diffracted and condensed on a quarter photo-detector (PD) or a photo-detector 10', thereby detecting a tracking error signal and a focus error signal.
JP-A-6-295481 discloses an optical pickup comprising a semiconductor substrate including a semiconductor laser and photo-detector means having a plurality of light detectors, a collimator lens, an objective lens, light path splitting means interposed between the collimator lens and the objective lens for polarizing and splitting the return light path from the magneto-optical recording medium into first and second light paths, a first hologram arranged as a unit integrated with the semiconductor substrate for diffracting the return light from the first light path, a second hologram for diffracting the return light from the second light path, and a light splitter having a polarize/split surface, wherein the positive and negative lights linearly-diffracted by the first hologram and the light split by the polarize/split surface are detected by the photo-detector means.
In the optical system shown in FIG. 1 or the optical pickup disclosed in JP-A-6-295481, the effective utilization of the mounting space and the reduction in the number of parts is a critical problem. Specifically, an increased weight or an increased mounting space of the optical parts used for the optical pickup bring about a bulky head base for supporting them. The result is a higher cost of an apparatus having a disadvantageous performance and an uncompetitive price. Especially, the magneto-optical signal detection means including the Wollaston prism 7, the condenser lens 8 and the photo-detector 9 shown in FIG. 1 uses expensive optical parts with an isolated magneto-optical signal detector and thus has a disadvantageous configuration for packaging.